The present invention relates to an improved cushioning member and method of making the same, and more particularly to a fluid filled bladder having controlled flex tensile members which allows for the formation of complex-curved contours and shapes while minimizing the amount of surrounding foam material. The present invention also relates to footwear wherein the bladder with controlled flex tensile members is used as a cushioning device within a sole.
Considerable work has been done to improve the construction of cushioning members which utilize fluid filled bladders such as those used in shoe soles. Although with the recent developments in materials and manufacturing methods, fluid filled bladder members have greatly improved in versatility, there remain problems associated with obtaining optimum performance and durability. Fluid filled bladder members are commonly referred to as xe2x80x9cair bladders,xe2x80x9d and the fluid is generally a gas which is commonly referred to as xe2x80x9cairxe2x80x9d without intending any limitation as to the actual gas composition used.
Closed-celled foam is often used as a cushioning material in shoe soles and ethylene-vinyl acetate copolymer (EVA) foam is a common material. In many athletic shoes, the entire midsole is comprised of EVA. While EVA foam can easily be cut into desired shapes and contours, its cushioning characteristics are limited. One of the advantages of gas filled bladders is that gas as a cushioning compound is generally more energy efficient than closed-cell foam. This means that a shoe sole comprising a gas filled bladder provides superior cushioning response to loads than a shoe sole comprising only foam. Cushioning generally is improved when the cushioning component, for a given impact force, spreads the impact force over a longer period of time, resulting in a smaller impact force being transmitted to the wearer""s body. Even shoe soles comprising gas filled bladders include some foam, and a reduction in the amount of foam will generally afford better cushioning characteristics.
Some major engineering problems associated with the design of air bladders formed of perimeter barrier layers include: (I) obtaining complex-curved, contoured shapes without the formation of deep peaks and valleys in the cross section which require filling in or moderating with foams or plates; (ii) ensuring that the means employed to give the air bladder its complex-curved, contoured shape does not significantly compromise the cushioning benefits of air; and (iii) reducing fatigue failure of the bladders caused by cyclic folding of portions of the bladder.
The prior art is replete with attempts to address these difficulties, but often presenting new obstacles in the process of addressing these problems. Most of the prior art discloses some type of tensile member. A tensile member is an element associated with the bladder which ensures a fixed, resting relation between the top and bottom barrier layers when the air bladder is fully inflated, and which often is in a state of tension while acting as a restraining means to maintain the general form of the bladder.
Some prior art constructions are composite structures of air bladders containing foam or fabric tensile members. One type of such composite construction prior art concerns air bladders employing an open-celled foam core as disclosed U.S. Pat. Nos. 4,874,640 and 5,235,715 to Donzis. These cushioning elements do provide latitude in their design in that the open-celled foam cores allow for complex-curved and contoured shapes of the bladder without deep peaks and valleys. However, bladders with foam core tensile members have the disadvantage of unreliable bonding of the core to the barrier layers. FIGS. 1 and 2 illustrate a cross section of a prior art bladder 10 employing an open-celled foam core 12 as a tensile member. FIG. 2 illustrates the loaded condition of bladder 10 with load arrows 14. One of the main disadvantages of bladder 10 is that foam core 12 gives the bladder its shape and thus must necessarily function as a cushioning member which detracts from the superior cushioning properties of air alone. One reason for this is that in order to withstand the high inflation pressures associated with air bladders, the foam core must be of a high strength which requires the use of a higher density foam. The higher the density of the foam, the less the amount of available volume in the bladder for gas. Consequently, the reduction in the amount of gas in the bladder decreases the benefits of gas cushioning.
Even if a lower density foam is used, a significant amount of available volume is sacrificed which means that the deflection height of the bladder is reduced due to the presence of the foam, thus accelerating the effect of xe2x80x9cbottoming out.xe2x80x9d Bottoming out refers to the premature failure of a cushioning device to adequately decelerate an impact load. Most cushioning devices used in footwear are non-linear compression based systems, increasing in stiffness as they are loaded. Bottoming out is the point where the cushioning system is unable to compress any further. Also, the elastic foam performs a significant portion of the cushioning function and is subject to compression set. Compression set refers to the permanent compression of foam after repeated loads which greatly diminishes its cushioning aspects. In foam core bladders, compression set occurs due to the internal breakdown of cell walls under heavy cyclic compression loads such as walking or running. The walls of individual cells constituting the foam structure abrade and tear as they move against one another and fail. The breakdown of the foam exposes the wearer to greater shock forces.
Another type of composite construction prior art concerns air bladders which employ three dimensional fabric as tensile members such as those disclosed in U.S. Pat. Nos. 4,906,502 and 5,083,361 to Rudy, which are hereby incorporated by reference. The bladders described in the Rudy patents have enjoyed considerable commercial success in NIKE, Inc. brand footwear under the name Tensile-Air(copyright) and Zoom(trademark). Bladders using fabric tensile members virtually eliminate deep peaks and valleys, and the methods described in the Rudy patents have proven to provide an excellent bond between the tensile fibers and barrier layers. In addition, the individual tensile fibers are small and deflect easily under load so that the fabric does not interfere with the cushioning properties of air.
One shortcoming of these bladders is that currently there is no known manufacturing method for making complex-curved, contoured shaped bladders using these fabric fiber tensile members. The bladders may have different heights, but the top and bottom surfaces remain flat with no contours and curves. FIGS. 3 and 4 illustrate a cross section of a prior art bladder 20 employing a three dimensional fabric 22 as a tensile member. FIG. 4 illustrates the loaded condition of bladder 20 with load arrows 24. As can be seen in FIGS. 3 and 4, the surfaces of bladder 20 are flat with no contours or slopes.
Another disadvantage is the possibility of bottoming out. Although the fabric fibers easily deflect under load and are individually quite small, the sheer number of them necessary to maintain the shape of the bladder means that under high loads, a significant amount of the total deflection capability of the air bladder is reduced by the volume of fibers inside the bladder and the bladder can bottom out.
One of the primary problems experienced with the fabric fibers is that these bladders are initially stiffer during initial loading than conventional gas filled bladders. This results in a firmer feel at low impact loads and a stiffer xe2x80x9cpoint of purchasexe2x80x9d feel than belies their actual cushioning ability. This is because the fabric fibers have a relatively low elongation to properly hold the shape of the bladder in tension, so that the cumulative effect of thousands of these relatively inelastic fibers is a stiff effect. The tension of the outer surface caused by the low elongation or inelastic properties of the tensile member results in initial greater stiffness in the air bladder until the tension in the fibers is broken and the solitary effect of the gas in the bladder can come into play which can affect the point of purchase feel of footwear incorporating bladder 20. The Peak G curve, Peak G v. time in milliseconds, shown in FIG. 5 reflects the response of bladder 20 to an impact. The portion of the curve labeled 26 corresponds to the initial stiffness of the bladder due to the fibers under tension, and the point labeled 28 indicates the transition point in which the tension in the fibers of fabric 22 are xe2x80x9cbrokenxe2x80x9d and give way to more of the cushioning effects of the air. The area of the curve labeled 30 corresponds to loads which are cushioned with more compliant gas. The Peak G curve is a plot generated by an impact test such as those described in the Sport Research Review, Physical Tests, published by NIKE, Inc. as a special advertising section, January/February 1990, the contents of which is hereby incorporated by reference.
Another category of prior art concerns air bladders which are injection molded, blow-molded or vacuum-molded such as those disclosed in U.S. Pat. No. 4,670,995 to Huang and U.S. Pat. No. 4,845,861 to Moumdjian, which are incorporated herein by reference. These manufacturing techniques can produce bladders of any desired contour and shape while reducing deep peaks and valleys. The main drawback of these air bladders is in the formation of stiff, vertically aligned columns of elastomeric material which form interior columns and interfere with the cushioning benefits of the air. These bladders are designed to support the weight of the wearer. FIGS. 6 and 7 illustrate cross sections of a prior art bladder 40 which is made by injection molding, blow-molding or vacuum-forming with vertical columns 42. FIG. 7 illustrates bladder 40 in the loaded condition with load arrows 44. Since these interior columns are formed or molded in the vertical position, there is significant resistance to compression upon loading which can severely impede the cushioning properties of the air.
In Huang ""995 it is taught to form strong vertical columns so that they form a substantially rectilinear cavity in cross section. This is intended to give substantial vertical support to the cushion so that the cushion can substantially support the weight of the wearer with no inflation. Huang ""995 also teaches the formation of circular columns using blow-molding. In this prior art method, two symmetrical rod-like protrusions of the same width, shape and length extend from the two opposite mold halves to meet in the middle and thus form a thin web in the center of a circular column. These columns are formed of a wall thickness and dimension sufficient to substantially support the weight of a wearer in the uninflated condition. Further, no means are provided to cause the columns to flex in a predetermined fashion which would reduce fatigue failures. Huang""s columns are also prone to fatigue failure due to compression loads which force the columns to buckle and fold unpredictably. Under cyclic compression loads, the buckling can lead to fatigue failure of the columns.
FIG. 8 shows a close-up view of a prior art column similar to those shown in Huang with a thin web in the middle of the column halves formed by a center weld W and a slight draft angle xcex8 to the column halves. While Huang""s columns do not appear to have a draft angle, the commercial embodiments of the bladder taught by Huang have shown a draft angle similar to that shown in FIG. 8.
Included in this prior art category of molded bladders are bladders having inwardly directed indentations as disclosed in U.S. Pat. No. 5,572,804 to Skaja et al, which is hereby incorporated by reference. Skaja et al. disclose a shoe sole component comprising inwardly directed indentations in the top and bottom members of the sole components. Support members or inserts provide some controlled collapse of the material to create areas of cushioning and stability in the component. The inserts are configured to extend into the outwardly open surfaces of the indentations. The indentations can be formed in one or both of the top and bottom members. The indented portions are proximate to one another and can be engaged with one another in a fixed or non-fixed relation. In the Skaja patent, indentations that are generally hemispherical in shape and symmetrical about a central orthogonal axis are taught. The outside shape of the indentation, that is, the shape outlined at the surface of the bladder component is circular. The inserts have the same shape as the indentations. The hemispherical indentations and mating support members or inserts respond to compression by collapsing symmetrically about a center point. While the hemispherical indentations and inserts of Skaja provide for some variation in cushioning characteristics by placement, size and material, there is no provision for biasing or controlling the compression or collapse in a desired direction upon loading. The indentations and the mating inserts contribute to the cushioning response of the bladder which is opposed to the goal of the present invention in which the controlled collapse members are engineered specifically to not interfere with the cushioning response of gas or air.
Yet another prior art category concerns bladders using a corrugated middle film as an internal member as disclosed in U.S. Pat. No. 2,677,906 to Reed which describes an insole of top and bottom sheets connected by lateral connection lines to a corrugated third sheet placed between them. The top and bottom sheets are heat sealed around the perimeter and the middle third sheet is connected to the top and bottom sheets by lateral connection lines which extend across the width of the insole. An insole with a sloping shape is thus produced, however, because only a single middle sheet is used, the contours obtained must be uniform across the width of the insole. By use of the attachment lines, only the height of the insole from front to back may be controlled and no complex-curved, contoured shapes are possible. Another disadvantage of Reed is that because the third, middle sheet is a continuous sheet, all the various chambers are independent of one another and must be inflated individually which is impractical for mass production.
The alternative embodiment disclosed in the Reed patent uses just two sheets with the top sheet folded upon itself and attached to the bottom sheet at selected locations to provide rib portions and parallel pockets. The main disadvantage of this construction is that the ribs are vertically oriented and similar to the columns described in the patents to Huang and Moumdjian, and would resist compression and interfere with and decrease the cushioning benefits of air. As with the first embodiment of Reed, each parallel pocket thus formed must be separately inflated.
A prior bladder and method of construction using flat films is disclosed in U.S. Pat. No. 5,755,001 to Potter et al, which is hereby incorporated by reference. The interior film layers are bonded to the envelope film layers of the bladder which defines a single pressure chamber. The interior film layers act as tensile members which are biased to compress upon loading. The biased construction reduces fatigue failures and resistance to compression. The bladder comprises a single chamber inflated to a single pressure with the tensile member interposed to give the bladder a complex-contoured profile. There is, however, no provision for multiple layers of fluid in the bladder which could be inflated to different pressures providing improved cushioning characteristics and point of purchase feel.
Another well known type of bladder is formed using blow molding techniques such as those discussed in U.S. Pat. No. 5,353,459 to Potter et al, which is hereby incorporated by reference. These bladders are formed by placing a liquefied elastomeric material in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized gas is introduced. The pressurized gas forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold to form a bladder having the preferred shape and configuration.
There exists a need for an air bladder with a suitable tensile member which solves all of the problems listed above: complex-curved, contoured shapes; elimination of deep peaks and valleys; no interference with the cushioning benefits of air alone; and the provision of a reliable bond between tensile member and outer barrier layers. As discussed above, while the prior art has been successful in addressing some of these problems, they each have their disadvantages and fall short of a complete solution.
The present invention pertains to a bladder with controlled flex connecting members extending between the top and bottom outer layers of bladder. The bladder of the present invention may be incorporated into a sole assembly of an article of footwear to provide cushioning. When pressurized, the outer layers are placed under tension, and the connecting members function as tension members. The bladder provides a reliable bond between the tensile members and the outer barrier layers, and can be constructed to have complex-curved, contoured shapes without interfering with the cushioning properties of air. A complex-contoured shape refers to varying the surface of the bladder in more than one direction. The present invention overcomes the enumerated problems with the prior art while avoiding the design trade-offs associated with the prior art attempts.
In accordance with one aspect of the present invention, a bladder is formed by blow-molding or rotational molding. Both of these methods create internal connection/tensile members which are integral with the outer perimeter layer. Since the outer perimeter and the internal tensile members are formed at the same time and of the same material, bonding problems between layers is eliminated and manufacturing is simplified. By utilizing pins in the blow-molded or rotational mold, tensile column members are formed which can provide a finely contoured shape, but which do not significantly interfere with the cushioning properties of the air, when the bladder contains air or another fluid. It is desirable that the tensile members compress easily under relatively low loads, those exceeding xc2xd body weight (35 kg) and preferably below 25 kg. In order to prevent fatigue stress on the members, a predetermined flex point is molded into at least a portion of each column. This assures that the members will flex under relatively low loads and that the flexure will occur in a predictable manner, eliminating the prior art problem of fatigue failure in the vertical columns.
To ensure that the tensile members do not interfere with the cushioning properties of air they are configured to be sufficiently flexible to receive compressive loads but are durable even under repeated loading. Broadly, there are two configurations: one in which the tensile member is constructed to collapse upon compressive loading, and one in which the tensile member is constructed to bend or fold like a hinge upon compressive loading in a predetermined location.
In another aspect of the present invention the shape of the flexible tensile column members and the interface at the flex point are manipulated to assist in finely tuning the cushioning properties of the final bladder. Differently shaped cross-sections of columns, e.g. circles, ovals, squares, rectangles, triangles, spirals, half-moons, helices, etc., impart different amounts of resistance to compression and exhibit varying flex properties. Also, the placement, thickness and number of flex points can significantly effect the bending, collapsing, or folding properties of the tensile members. For example, multiple accordion-like pleats molded into the columns impart more flexibility than a single notch or pleat of the same thickness. Additionally, the columns need not be arranged perpendicular to the plane of the bladder surface. By forming the tensile members at various angles, the direction that the tensile member bends or folds can be further controlled.
Yet another aspect of the invention is to vary the lengths of the opposing ends of the tensile columns by utilizing pin or rod-like protrusions of different lengths in the mold, the joint or hinge in the tensile members can be formed off-center. The longer of the two pin or rod-like protrusions forms a column portion of longer length than the shorter pin or rod-like protrusion. This variation in the tensile column""s length can be manipulated to direct the flexing of the column under compression.
In another embodiment, the flex point of the tensile column is manipulated by altering the cross-section size of the pin or rod-like protrusions in the mold, whereby the pins or rod-like protrusions in one mold half are larger in cross-section than the ones in the opposing half. This produces a tensile column with one portion larger than the other which allows the smaller portion of the column to telescope or nest into the larger portion upon loading. In such a construction, the larger portion collapses around the smaller portion, rather than acting as a hinge.
In yet another embodiment, spring elements such as elastomeric sheets, may be insert-molded during the blow-molding process to direct the flex properties of the columns. For example, a thin strip of thermoplastic urethane of the same type used to form the main bladder can be located in the mold in such a way that it spans the gap between two of the columns forming pins or rod-like protrusions located in the same half of the mold. The resulting columns formed would be tied together horizontally in the center web portion by the strip. This would prevent columns from flexing easily in any direction except inwardly toward the shared strip.
Another method of manipulating the flex properties of the tensile columns is to vary the draft angle of the pins or rod-like protrusions in the mold which form the columns. A draft angle of zero degrees would produce a column with essentially vertical walls. A draft angle of 5xc2x0 to 45xc2x0 is needed in order to cause the column to flex in a predictable manner. In general an increased draft angle in combination with another structural difference such as asymmetry will provide the desired predicted location of collapse. Engineering the location of collapse or flexure in this manner prevents the failures noted with prior art devices. By manipulating some or all of the above factors in various combinations, cross-sectional size, length, shape, hinges, thickness, draft angles and symmetry, it is possible to finely tune the cushioning properties of the bladder and select the most appropriate flex characteristic to prevent fatigue failures and prevent the tensile columns from significantly detracting or interfering with the cushioning benefits and feel of the air.
The present invention provides a bladder with tensile members of complex-curved, contoured shapes without deep peaks and valleys, which facilitates utilization of the cushioning properties of air and which provides a reliable bond between the tensile members and the outer barrier layers of the bladder. The tensile members are columns formed integrally with the barrier layer and are formed with predetermined flex points which are constructed to flex upon compression by collapsing, bending, or rolling so that the tensile members do not substantially interfere with the cushioning effects of the air. The tensile members are less susceptible to fatigue failures when they are not required to perform a significant supportive function and the flex point is constructed for taking repeated compressive loads. This configuration ensures that the tensile members will not compromise the cushioning properties of air.
These and other features and advantages of the invention may be more completely understood from the following detailed description of the preferred embodiment of the invention with reference to the accompanying drawings.